The invention relates to a method defined more closely in the preamble of the claim 1, for generating over-pressure gas, especially pressurized air.
In industry, when repairing buildings etc, pressurized air is required in which the pressure is typically within the order of 7 to 10 bar of over-pressure. To generate this pressurized air, piston or screw compressors are conventionally used which are usually oiled. The aim is to separate oil mist from the pressurized air with which it is mixed, but at least some oil remains in the pressurized air and thus ends up in the environment. Conventional piston or screw compressors are large in size, heavy, and often also noisy. They require maintenace relatively often, and especially in uncooled screw compressors, the operating efficiency is often poor.
Kinetic compressors have conventionally been used to produce pressurized air in the above-described manner only if the amount of air required has been substantial, for otherwise it would have been necessary to use a very high rotational speed. It is difficult to generate a very high rotational speed with an overdrive gear; the overdrive gear becomes expensive and is liable to failures.
U.S. Pat. No. 5,065,590 presents a cooling apparatus using a small-sized high-speed centrifugal compressor. This compressor is driven with an electromotor, whose rotor shaft is equipped with compressor stages. In between the compressor stages there is an intermediate cooler. This compressor shaft according to the patent publication is supported with ball bearings in which one drawback is, for instance, that they require heavy oiling at high speeds; otherwise the friction in them causes the bearings to heat up and wear down rapidly.
International patent application WO 94/29597 presents a cooling compressor of a centrifugal type. This compressor comprises one or more compressor stages, an electromotor, and a shaft which is supported with radial bearings, such as magnetic or gas bearings. Furthermore, the shaft is provided with axial locating means. To control the function of these locating means, the compressor is equipped with sensors, by means of which the location of the rotor at a given time can be detected.
In kinetic compressors of prior art, the rotational speed of the electromotor can be maintained substantially constant. The pressure and output of the compressor are adjusted by means of adjustable guide vanes.
The present invention relates to a method which is implemented with a device which generates pressurized air with a very rapidly rotating kinetic compressor based on high-speed technology. In this context, high speed technology denotes that the rotor unit is directly coupled to a high-speed electric machine, which is supplied with the necessary frequency generated by an inverter. Because the device applying the invention is not equipped with an overdrive gear, it is possible to use very high rotational frequencies witout problems. Preferably, the inverter is provided with a control logic for the compressor.
To attain the aforementioned purposes, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the appended claim 1.
The other independent claims present some advantageous embodiments of the invention.